Género

So far we ignored the backreaction of the fields onto the curvature of spacetime. To cure this we refine our ansatz such that the metric of the ordinary (3+1)- dimensional spacetime is given by a FRIEDMANN ROBERTSON WALKERcosmology with scale factora(t)and HUBBLEparameterH(t). The equations of motion now

read ¨ T=−3HT˙ −4Σ˙ ΣT˙ − Σ−2_{−1T, (A.25a)} ¨ Σ=−3HΣ˙ +gΣ2 ˙T2Σ2+2Σ2−1T2, (A.25b) 4 ˙H=−6H2+Σ˙2+gΣ2Σ2T˙2+T2Σ2−1, (A.25c) 0=6H 2 Σ2 + ˙ Σ2 Σ2 +g Σ2_T˙2_+T2_Σ2_{−1. (A.25d)}

An exponentially decaying ansatz for the tachyon field T ∝ e−t at large t now induces an asymptotical solution, where the modulusΣ ∝ 1_t tends to zero and the curvatureH = ₃2_t follows the evolution of a flat dust-filled universe. Unfortunately this solution is incompatible with the constraint equation (i. e. the 00-component of the field equations).

There are many people and institutions to whom I am very much indebted. First of all, I want to thank Ivo SACHSfor supervising this thesis and thus allowing me to learn from him. I am enormously indebted to him for all the support and advice he gave, as well as the opportunity to take part in teaching theoretical physics to students.

I would like to thank Viatcheslav MUKHANOVfor the possibility to work in the cosmology and astro-particle physics group at the ARNOLDSOMMERFELDCENTER for theoretical physics of LUDWIG-MAXIMILIANS-UNIVERSITÄT. Especially for the chapter on limiting the curvature through higher derivatives I am grateful for the initial collaboration with him and for helpful discussions throughout the completion of the paper.

For an enjoyable atmosphere and administrative support of all kind, I have to thank the group’s secretary Herta WIESBECK-YONIS.

I have also drawn up a list of people from all of whom I have learned a lot of physics and who all have helped me in one way or another during my P. h. D. studies, which is probably incomplete for which I apologise in advance, in al- phabetical order: Nikolas ÅKERBLOM, Murad ALIM, Frederik BEAUJEAN, Stefan BEREZ, Otmar BIEBEL, Gabriel Lopes CARDOSO, Dorothea DEEG, Johanna ERD- MENGER, Michael HAACK, Robert HELLING, Tobias HOFBAUR, Matthias KAMINSKI, Jin U KANG, Dominique LÄNGE, Adrian MERTENS, Nicolas MOELLER, Sebastian MOSTER, Michael NIJS, Johannes OBERREUTER, Jan PERZ, Sebastian SCHLICHT, Maximilian SCHMIDT-SOMMERFELD, Franziska SYNATSCHKE-CZERWONKA, Vitaly VANCHURIN, Alexander VIKMAN, Sergei WINITZKI, and Marco ZAGERMANN.

For a wonderful introductory lecture on string theory given at the String-Steilkurs in 2006 at DESY in Hamburg, I would like to thank Volker SCHOMERUS. His lecture and the notes we took there mainly inspired section 2.2.

For three years of financial support and many inspiring events like the collo- quium, several workshops and especially the YOUNG SCIENTIST WORKSHOPat Ringberg castle I would like to thank the INTERNATIONAL MAX PLANCK RE- SEARCH SCHOOLfor Particle Physics at the MAXPLANCKINSTITUTE FORPHYSICS. Also, thanks are due to the current, respectively the former coordinator of the IMPRS for Particle Physics, Frank STEFFENand Jochen SCHIECK, as well as to the secretaries Andrea HAFTELand Franziska RUDERT.

Furthermore I want to thank Jonathan FINLEY, the dean of studies at TECH- NISCHEUNIVERSITÄT MÜNCHENfor accepting my working on this thesis while already employed as his assistant, and Joachim DIENERfor constantly reminding me to complete the thesis.

Such work is greatly alleviated if one can rely on friends and family and thus I warmly thank my parents and brothers for continued support and I duly apologise for the lack of presence to my nices and nephew.

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